In recent years, the number of mobile radio terminals to be loaded to a mobile station, particularly, a vehicle station is increased with rapid progress thereof toward high level information systems. The terminals may be a GPS (Global Positioning System) receiver, mobile telephone system and ETC (electronic toll collection) communication system. In these radio terminals, different frequencies are respectively used to eliminate interference. Therefore, the radio terminals are required to have respective antennas which operate, that is, resonate in different frequencies.
Moreover, it is desirable that these antennas are installed at the area near the instrument panel of vehicle or on the vehicle chassis in which rather excellent radio wave propagation condition can be assured. Moreover, it is also requested to install the antenna within the instrument panel or within a rear view mirror in a vehicle compartment, considering the external appearance of vehicle, acquisition of sufficient visual field for driver and safe drive and operation for vehicle.    (1) It is difficult to install a plurality of antennas in the limited space within a vehicle. Particularly, since air-conditioner, various meters, air-bag apparatus, moreover, information terminal devices such as audio device and navigation device are provided within the instrument panel, it is very difficult to provide a space for installation of antennas.    (2) Cables of the same number as the number of radio terminals are required for connection between the radio terminal and antenna.    (3) Many metallic members exist within the instrument panel to form metal cabinets of various devices and vehicle body. The reflected waves from these metallic members and the direct wave radiated directly from the antenna are complicatedly interfere with each other and thereby many dead-band directions of radio waves. That is, many null points are formed and the antenna characteristics are worsened.
A multiple-frequency antenna covering a plurality of resonant frequencies has been developed as a means for solving the above problems (1) and (2). For example, JP-A-2000-68736 discloses an inverse F-antenna which is composed of three unit-radiation-conductors of different lengths arranged keeping the predetermined interval for operation in three frequency bands. Moreover, U.S. Pat. No. 6,112,102 (Japanese PCT Publication No. 2001-501412, WO 98/15028) discloses, as a multiple-frequency antenna in the other structure, a helical antenna combining two helical antennas of different pitches. Further, JP-A-2000-59130 discloses an antenna combining a linear conductor bar and a helical antenna. However, these multiple-frequency antennas of the prior art cannot solve the above problem (3).
The problem (3) arises, as is well known, when the direct wave radiated from a radiation element of the antenna interfere with the wave generated when a surface current flowing on the ground plane of the antenna is re-radiated from the end part of the ground plane.
U.S. Pat. No. 6,262,495 and the publication, “Antenna on High-Impedance Ground Planes, by D. Sievenpiper, et. al., IEEE MTT-S Digest, WEF1-1, 1245 (1999), disclose an antenna for solving the problem (3). That is, a ground plane called the high impedance ground plane (HIP) is used as shown in FIGS. 1A and 1B. In this HIP, hexagonal small metal plates 4 are periodically and two-dimensionally disposed on the surface of a dielectric material layer 3, and these metal plates 4 are coupled with a metal plate 2 at the rear surface of the dielectric material layer 3 and a through-hole 5 as a linear metal bar. Thus, a gap between the adjacent hexagonal small metal plates 4 forms a capacitance element. A current route of the end part of the hexagonal small metal plate 4→through-hole 5→metal plate 2→through-hole 5→end part of small metal plate 4 forms an inductance element. An LC parallel resonant circuit is formed with adjacent units consisting of these capacitance and inductance elements. A substrate having a higher impedance characteristic in the LC resonant frequency, that is, the HIP can be completed by forming many LC parallel resonant circuits on the metal plate 2.
The HIP can be thought of a kind of the photonic band gap material or the photonic band gap structure (PBG). PBG means a material or a structure in which a frequency region (called a band gap) which prohibits propagation of an electromagnetic wave of the particular frequency, that is, propagation of the surface current at the inside or on the surface by introducing the structure where two kinds of different substances such as dielectric material and metal are orderly arranged in two or three dimensions with the period in the order of wavelength. The band gap is formed in the particular structure for the electromagnetic wave of microwave band and light wave.
The above HIP is in the PBG structure corresponding to the electromagnetic wave covering from the microwave band to the millimeter wave band and has the following two characteristics.                One is that the electromagnetic waves entering the HIP are reflected in the same phase in the resonant frequency. These waves are reflected in the inverse phase in the case of the ordinary metal plate.        The other is that a surface current of the resonant frequency and the frequency element near this resonant frequency does not flow into the HIP.        
The above IEEE publication shows the result of comparison of antenna characteristics when a monopole antenna of the same size is installed on a metal plate or on the HIP. That is, in the former case, since a surface current is generated, the direct wave and the wave radiated from the end part of the metal plate interferes with each other in the upper surface direction to generate a ripple in the directivity of antenna and a large amount of radiation in the lower surface direction can also be generated. On the other hand, in the latter case, since a surface current does not flow, radiation from the end part is never generated. Therefore, ripple in the directivity is not generated in the upper surface direction and radiation in the lower surface direction is also reduced.
As such, the above problem (3) can be solved by utilizing the HIP as the ground plane of antenna. However, this prior art cannot solve the above problems (1) and (2).